Broadcasting over a struct with two arrays

[mateuszbaran]

Hi!

I have a type that looks like

struct A{X1<:AbstractArray, X2<:AbstractArray}
    x1::X1
    x2::X2
end
a = A([1, 2], [1 2; 3 4])
b = A([1, 4], [-1 2; -3 4])

and I’d like to extend broadcasting such that, for example, a .= b .- a is equivalent to

a.x1 .= b.x1 .- a.x1
a.x2 .= b.x2 .- a.x2

Is there any example I can follow to do this? I don’t need to combine broadcasting of objects of type A with objects of other types.

[francesco.alemanno]

you should switch to “mutable struct” then.

[mateuszbaran]

I don’t need to change x1 or x2, only their content.

[simeonschaub]

Have you seen https://docs.julialang.org/en/v1/manual/interfaces/#man-interfaces-broadcasting-1?

[mateuszbaran]

Yes, I’ve seen that and it was very helpful when I just needed to forward broadcasting to a single wrapped array (although even then I had to look at the code of StaticArrays to make it work well because for example it does not mention that it’s a good idea to add a new method to Base.dataids like here: StaticArrays.jl/SizedArray.jl at b150a8c0eeca4b886a02f2ffb16406653a6aa054 · JuliaArrays/StaticArrays.jl · GitHub ). That’s why I was hoping to just work using a full existing implementation.

[mbauman]

Check out

Customizing Broadcasting Recursively for Collections of Arrays

I previously asked this question and am aware of the section in the manual about customizing broadcasting. However, rather than defining a new type which behaves like a single array, I would like to define broadcasting recursively for types which contain several arrays (or types which are themselves collections of arrays). That is, imagine we have struct A{T so that broadcasting is already defined for T} a::T b::T end If x::A, y::A, calculate z = x .+ y by calling z.a = x.a .+ y.a (etc.). …

[mateuszbaran]

Thanks, I didn’t notice that thread. I’ll try something out.

[mateuszbaran]

For future reference: I finally managed to do what I wanted to do here: Copy and broadcasting for ProductRepr by mateuszbaran · Pull Request #336 · JuliaManifolds/Manifolds.jl · GitHub and it works well enough for me. I hope it’s a good reason to post in this (very old) thread .